Flame retarding additives for use in polymeric compositions include phosphates, phosphoric acid esters and thio-phosphoric acid esters containing halogenated alkyl radicals. Also, metal salts notably sulfonate salts of alkali or alkaline earth metals have been disclosed to improve the flame retardance of polycarbonates. Further, chlorine or bromine substitutions in some of the phenolic diols used in the preparation of polycarbonates have been disclosed to impart flame retardance to the polycarbonate resin.
Illustrative of the sulfonic acid salts and of metal salts are those disclosed in U.S. Pat. Nos. 3,775,367; 4,067,846; 4,073,678; 4,075,164; 4,066,618; 4,069,201; 4,093,589; 3,971,756; 3,933,734; 3,940,366, and 3,919,167.
Some of these flame retardants, however, in order to be effective, are added in relatively large amounts such as to adversely effect some of the desirable properties of the base resin. For example, both the impact strength and the hydrolytic stability are compromised upon addition of large quantities of salt. Further, many of these flame retarding salts are susceptible to volatilizaton at the high molding temperatures of polycarbonates, necessitating thus the addition of excess amounts of salt which in turn bring about haze and loss of transparency. Since there is no uniformity of processing conditions among molders, it becomes difficult, if not impossible, to regulate the ultimate amount of salt incorporated into the resins.
Flame retardants incorporating a phthalimide group have been disclosed in, for instance, British Pat. No. 1,287,934 and U.S. Pat. Nos. 3,873,567; 3,923,734; 3,915,930; 3,868,388; 4,087,441; 4,001,179; 4,003,862 and 4,320,049.